hile several classes of viruses form stable associations with heir hosts by integrating one or more copies of their genomes nto the host cell DNA, retroviruses provide a unique and mportant system for the study of integrative recombination. etroviral genomes are integrated with high efficiency at pecific sites within the viral genome, but at a large number f sites in the host chromosome. Often a consequence of this ntegration event is a readily detectable change in cell rowth. Modern methods of molecular cloning and analysis allow or the detection and amplification of rare DNA sequences such s an integrated viral segment. Molecular clones of several ewly integrated retroviral genomes were produced in either lasmid or bacteriophage cloning vehicles using recombinant DNA echniques and were characterized using electron microscope eteroduplex and R-loop methods. Detection of sequence omology even when interrupted by intervening cellular DNA is ften accurately mappable in the electron microscope using hese methods. These studies have not only shown the rrangement of integrated viral sequences within infected host ell DNA; but have also demonstrated the presence and sequence rrangement of certain viral transforming sequences within ormal, uninfected host cells as well. The major objective of hese studies has been the application of physical and iochemical techniques to assess the influence of integrative osition or flanking cellular sequences on subsequent viral unction and to define in molecular terms those events which ake place during integrative recombination.